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You only want to use a "USB flash drive" to make a bootable installer.
I'd suggest at least 16gb in size.
Don't use it for a "regular booting drive"
Use an SSD for that !
 
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I just finished cloning my 2017 21" iMac (1TB spinner) using Fishrrman's directions and it is now better than new. I picked up a 1TB Sandisk Extreme SSD ($140 @ Costco) The cloning took a little over 3 hours for the 350GB used on the original harddrive.

The difference in performance is dramatic. System Preferences appears instantaneously. MS Office was practically unusable before... 52 icon "bounces" + 12 more seconds to get MS Word up. Now, 4 bounces and it appears. I'm checking to make sure that Time Machine is backing up this new drive rather than the internal drive...but other than that, I'm ready to rock n roll.

This is the 2nd iMac I've done this to. (the first, a 2013 21", has been re-purposed to be my Plex server 2 years ago and it is still speeding along) For anyone who is considering doing this, I highly recommend it. The process is dead simple, the cost of quality drives has come way down, and the benefits are well worth it.
 
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